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IndVarSimplify: Allow LFTR to fire more often

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I added a pessimization in r217102 to prevent miscompiles when the
incremented induction variable was used in a comparison; it would be
poison.

Try to use the incremented induction variable more often when we can be
sure that the increment won't end in poison.

Differential Revision: http://reviews.llvm.org/D6222

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222213 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-11-18 02:20:58 +00:00
parent 5d8a792eac
commit 1ade0f0faa
2 changed files with 23 additions and 2 deletions
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23
lib/Transforms/Scalar/IndVarSimplify.cpp
View File

@ -1719,8 +1719,29 @@ LinearFunctionTestReplace(Loop *L,
// FIXME: In theory, SCEV could drop flags even though they exist in IR.
// A more robust solution would involve getting a new expression for
// CmpIndVar by applying non-NSW/NUW AddExprs.
auto WrappingFlags =
ScalarEvolution::setFlags(SCEV::FlagNUW, SCEV::FlagNSW);
const SCEV *IVInit = IncrementedIndvarSCEV->getStart();
if (SE->getTypeSizeInBits(IVInit->getType()) >
SE->getTypeSizeInBits(IVCount->getType()))
IVInit = SE->getTruncateExpr(IVInit, IVCount->getType());
unsigned BitWidth = SE->getTypeSizeInBits(IVCount->getType());
Type *WideTy = IntegerType::get(SE->getContext(), BitWidth + 1);
// Check if InitIV + BECount+1 requires sign/zero extension.
// If not, clear the corresponding flag from WrappingFlags because it is not
// necessary for those flags in the IncrementedIndvarSCEV expression.
if (SE->getSignExtendExpr(SE->getAddExpr(IVInit, BackedgeTakenCount),
WideTy) ==
SE->getAddExpr(SE->getSignExtendExpr(IVInit, WideTy),
SE->getSignExtendExpr(BackedgeTakenCount, WideTy)))
WrappingFlags = ScalarEvolution::clearFlags(WrappingFlags, SCEV::FlagNSW);
if (SE->getZeroExtendExpr(SE->getAddExpr(IVInit, BackedgeTakenCount),
WideTy) ==
SE->getAddExpr(SE->getZeroExtendExpr(IVInit, WideTy),
SE->getZeroExtendExpr(BackedgeTakenCount, WideTy)))
WrappingFlags = ScalarEvolution::clearFlags(WrappingFlags, SCEV::FlagNUW);
if (!ScalarEvolution::maskFlags(IncrementedIndvarSCEV->getNoWrapFlags(),
SCEV::FlagNUW | SCEV::FlagNSW)) {
WrappingFlags)) {
// Add one to the "backedge-taken" count to get the trip count.
// This addition may overflow, which is valid as long as the comparison is
// truncated to BackedgeTakenCount->getType().

2
test/Transforms/IndVarSimplify/lftr-extend-const.ll
View File

@ -2,7 +2,7 @@
; CHECK-LABEL: @foo(
; CHECK-NOT: %lftr.wideiv = trunc i32 %indvars.iv.next to i16
; CHECK: %exitcond = icmp ne i32 %indvars.iv, 511
; CHECK: %exitcond = icmp ne i32 %indvars.iv.next, 512
define void @foo() #0 {
entry:
br label %for.body